Output drivers built in standard CMOS technologies vary greatly in their performance characteristics as a result of process variations, temperature variations, operating voltage variations, and output capacitive loading. These variations cause the delay time and the slew rate at the output of the pads to vary considerably. The emphasis of most designers has been in managing the slew rate of the output of input/output drivers. Techniques that have been used by designers include segmentation of the output driver and monitoring of variables that affect the output slew rate, including process, voltage, temperature and load capacitance. The output pad is then calibrated, based on the characteristics of these variables, and adjustments are made. Calibration circuitry is required and the calibration process takes additional time. Additional power, circuitry requiring silicon area, and a larger number of critically time control signals are required using this technique. This leads to greater engineering effort and product cost.
Segmentation has also been used in a process in which the device segments are driven at slightly different times from each other, such that the driving point impedance at the output node is adjusted as the voltage of the output changes. This method also requires a calibration loop for compensation of process, voltage, temperature and load capacitance variables. Again, this technique is expensive to implement.